Cathode manufacture



Patented July 30, 1940 UNITED STATES CATHODE lWIANUFACTURE StantonUmbreit, West Orange, N. J., assignor to Radio Corporation of America, acorporation of Delaware No Drawing. Application October 31, 1939, SerialNo. 302,264

Claims. (01. 250-275) My invention relates to cathodes for electrondischarge devices, particularly to cathodes of the type in which thebase metal carries an oxide coating usually of the alkaline earth metal5 oxides.

An object of my invention is to provide an oxide coated cathode having awrought core consisting predominantly of nickel and free of strongreducing agents for the oxide coating.

A more specific object of my invention is to provide an oxide coatednickel cathode that liberates only enough active metal to produce goodprimary emission with only negligible reverse current, and that isstrong yet easy and inexlo pensive to manufacture.

The usual oxide coated cathodes is a sleeve or wire of commercial nickelcoated with active metal oxides such as barium-strontium oxides. Theusual commercial nickels contain some carbon, and also other elementssuch as silicon, magnesium and manganese, which are added to the moltennickel during the metallurgical treatment of the nickel to bring aboutdeoxidation of the nickel. These deoxidizers are such strong reducingagents that the slight amount present in commercial nickel causesexcessive liberation of the active metal of the oxides at the usualoperating temperature of an oxide coated cathode made of commercialnickel. This excess active 'metal evaporates and deposits on the gridand.

anode electrodes in the tube and emits electrons that causeobjectionable noise, reverse currents and low efficiency.

The commercial nickel used for filament wire and cathode sleeves isusually made by melting nickel to which some strong reducing agents havebeen added to eliminate the oxygen and oxides formed during the meltingoperation and facilitate working of the metal. Considerable effort hasbeen made to add just the right amount of deoxidizer to eliminate theoccludedoxygen or oxides in the melt yet leave a slight residue toinsure complete deoxidation of the melt. Since the amount of oxides inthe molten metal is 45 never known exactly, it is impossible to add'justthe right amount of deoxidizing agent. Therefore, it is customary to adda slight excess of the deoxidizing agent to insure complete removal ofoxides.

50 According to my invention cathode cores or the strip for making suchcores may be made of nickel which has been melted and sufiicient amountsof deoxidizers such as carbon, silicon, magnesium, manganese andaluminum added to 55 insure easy working of the nickel into thin stripsand fine cathode wires without regard to the amount of residualdeoxidizers in the final strip or wire. According to my invention thewrought nickel cathode core containing the impurities which reduce theoxide coatings of the core is I5 fired in a slightly oxidizingatmosphere at a temperature and for a sufficient time to render theimpurities harmless probably by oxidation. I have found that nickelstrip wire or sleeves fired in a hydrogen atmosphere saturated withwater 10 vapor is well suited for the purpose of oxidizing theimpurities in the nickel. Wet hydrogen firing has been found to beeasily controlled so that the common elementary impurities in thenickel, such as silicon, magnesium and carbon, may be 15 oxidized andrendered harmless without oxidizing the nickel. Use is made, accordingto my invention, of the lesser afiinity of nickel for oxygen than theaffinity of silicon, magnesium and carbon for oxygen. Nickel strip .002"in thickness, 20 for example, fired at 1100 C. for one hour in wethydrogen containing 20 to 30% moisture conditions the strip for cathodesleeves which will not liberate excessive amounts of active metal fromthe coating. Firing temperature and moisture 25 content of theatmosphere of course determines the length of firing. Hydrogen mayconveniently be wet by bubbling it through water before it is introducedto the firing furnace, the temperature of the water being raised aboveroom tempera- .30 ture if it is desired to increase the percent ofmoisture in the hydrogen. The wet hydrogen firing may of course followthe final filament wire drawing operation or the cathode sleeve wrappingoperation. 35

' Reducing agents, such as silicon manganese or magnesium, may berendered inactive by this treatment of mild oxidation of the wroughtmetal if the reducing agents are elementary and soluble in the nickel sothat they are capable of 40 difiusing to the surface where they can beoxidized. This process is of course best carried out on thin strips orcathodes heated at a high temperature so that diffusion to the surfaceis rapid. Alternatively the metal may be heated in a bath which willoxidize the nickel less rapidly than its included impurities.

The cathode core prepared according to my novel method is substantiallyfree from strong reducing agents, yet is active and cooperates with theusual alkaline earth metal oxide coating, such as barium-strontium oxidecoating to produce good electron emission.

I claim:

1. The method of making oxide coated cathodes having a core of wroughtmetal, consisting predominantly of nickel, which comprises firing thecore in a slightly oxidizing atmosphere at a temperature and for a timesufiicient to render inactive any reducing agents in said metal andinsufiicient to oxidize said metal, and then covering said core withsaid oxide coating.

2. The method of making oxide coated cathodes which comprises firing inwet hydrogen a Wrought nickel core containing reducing agents until thereducing agents are rendered inactive to reduce alkaline earth metaloxides, and coating the core with alkaline earth metal oxides.

3. The method of making cathodes with a wrought core predominantly ofnickel coated with alkaline earth metal oxides and containing strongreducing agents for the coating comprising firing the core at anelevated temperature in a slightly oxidizing atmosphere to oxidize saidagents, the temperature being high enough to rapidly diffuse thereducing agents toward the surface of the core, and then coating thecore with said alkaline earth metal oxides.

4. The method of making oxide coated cathodes comprising firing awrought core of said cathode in wet hydrogen to oxidize the strongreducing agents of the core, and then applying the oxide coating to thecore.

5. The method of making cathodes with a core predominantly of nickelcoated with an alkaline earth metal oxide and containing in the corereducing agents for the oxide coating comprising heating the core in anoxidizing atmosphere only until most of said agents are oxidized, andthen coating said core with said alkaline earth metal oxide.

STANTON UMBREIT.

